Organosilicon Reagents Research Articles

Diastereoselective Coupling of 1,3-Diene, Ketone, and Organometallic Reagents by Nickel Catalyst: Stereoselective Construction of Tetrasubstituted Carbon Centers

Abstract A nickel-catalyzed three-component coupling of 1,3-diene, ketone, and organoboron or organosilicon reagents was investigated. While the coupling reaction using PhB(OH)2 afforded a 1,3-syn-substituted 4-penten-1-ol derivative as a single diastereomer, the reaction in the presence of tetraorganosilicon reagent under similar conditions exclusively produced the corresponding 1,3-anti isomer. In both reactions, a tetrasubstituted carbon center was constructed in a highly diastereoselective manner.

Functionalization and solubilization of inorganic nanostructures and carbon nanotubes by employing organosilicon and organotin reagents

Covalent functionalization of nanowires of TiO 2 , ZnO and Al 2 O 3 has been carried out by employing the organosilicon reagents aminopropyltriethoxysilane and hexadecyltrimethoxysilane (HDTMS). The presence of the organosilane coating was confirmed by electron microscopy, energy dispersive X-ray analysis (EDXA) and IR spectroscopy. HDTMS-coated oxide nanowires give stable dispersions in CCl 4 and toluene. Nanoparticles of these metal oxides as well as of CeO 2 and Fe 3 O 4 could be solubilized in non-polar solvents by functionalizing with HDTMS. Nanotubes and nanoparticles of BN could also be functionalized and solubilized with HDTMS. Organotin reagents have also been used to covalently functionalize oxide nanostructures and multi-walled carbon nanotubes, thereby producing stable dispersions in CCl 4 and toluene. The organotin reagents used were dibutyldimethoxytin and trioctyltinchloride. Covalent functionalization of nanostructures using organosilane and organotin reagents provides a general method applicable to large class of inorganic materials as well as carbon nanotubes and is likely to be useful in practice.

Nickel(0)-catalyzed diastereoselective three-component coupling of 1,3-dienes, aldehydes, and organometallic reagents: influence of organometallic reagents on diastereoselectivity

A nickel-catalyzed diastereoselective alkylative three-component coupling of 1,3-diene and aldehyde with organoboron or organosilicon reagents has been realized. The diastereoselectivity was dramatically changed depending on the class of organometallic reagents. The reaction using ArB(OH) 2 in the presence of PPh 3 afforded 1,3- syn-substituted 4-penten-1-ol derivative as a single diastereomer. On the other hand, the coupling reaction with tetraorganosilicon reagent using NHC as a ligand under similar conditions exclusively produced the corresponding 1,3- anti isomer.

Vinylation of Aromatic Halides Using Inexpensive Organosilicon Reagents. Illustration of Design of Experiment Protocols

The preparation of styrenes by palladium-catalyzed cross-coupling of aromatic iodides and bromides with divinyltetramethyldisiloxane (DVDS) in the presence of inexpensive silanolate activators has been developed. To facilitate the discovery of optimal reaction conditions, Design of Experiment (DoE) protocols were used. By the guided selection of reagents, stoichiometries, temperatures, and solvents, the vinylation reaction was rapidly optimized with three stages consisting of ca. 175 experiments (of a possible 1440 combinations). A variety of aromatic iodides undergo cross-coupling at room temperature in the presence of potassium trimethylsilanoate using Pd(dba)2 in DMF in good yields. Triphenylphosphine oxide is needed to extend catalyst lifetime. Application of these conditions to aryl bromides was accomplished by the development of two complementary protocols. First, the direct implementation of the successful reaction conditions using aryl iodides at elevated temperature in THF provided the corresponding styrenes in good to excellent yields. Alternatively, the use of potassium triethylsilanolate and a bulky "Buchwald-type" ligand allows for the vinylation reactions to occur at or just above room temperature. A wide range of bromides underwent coupling in good yields for each of the protocols described.

Hexamethyldisilazane-Mediated Controlled Polymerization of α-Amino AcidN-Carboxyanhydrides

Ring-opening polymerizations of α-amino acid N-carboxyanhydrides (NCAs) initiated with amines typically form polypeptides with uncontrolled molecular weights and broad molecular weight distributions. However, we found that hexamethyldisilazane (HMDS)-mediated controlled NCA polymerizations gave polypeptides with predictable molecular weights and narrow molecular weight distributions. Using MS, NMR, and FT-IR, we demonstrated that the initiation step involved the cleavage of the N−Si bond of HMDS and the formation of a trimethylsilyl carbamate (TMS-CBM) terminal group. Polypeptide chains were propagated through the migration of TMS of the TMS-CBM end group to the incoming monomer and formed a new TMS-CBM terminal group. This organosilicon reagent mediated NCA polymerization offers a metal-free strategy for the convenient synthesis of homo- or block polypeptides with predictable molecular weights and narrow molecular weight distributions.

Silicone as an organosilicon reagent for the palladium-catalyzed cross-coupling reaction

Silicone, poly(diorganosiloxane), serves as an organosilicon reagent for palladium-catalyzed cross-coupling reactions. Treatment of poly[(aryl)methylsiloxane], poly[(alkenyl)methylsiloxane], or cyclic oligosiloxanes with various aryl iodides in the presence of silver(I) oxide (Ag 2O) or tetrabutylammonium fluoride (TBAF) and a catalytic amount of palladium affords the corresponding cross-coupling product in a good to excellent yield. The reaction of silicone with aryl chlorides in the presence of K 2CO 3/H 2O as an activator proceeded to afford biaryl derivatives in moderate to excellent yields. A wide range of aryl chlorides bearing an electron-donating or electron-withdrawing substituent on the aromatic ring are tolerated.

A New Catalytic Route for the Activation of sp‐Hybridized Carbon–Hydrogen Bonds

Vinyl-substituted silicon compounds react selectively with terminal alkynes in the presence of complexes containing [Ru]H and [Ru]Si bonds to form functionalized silylethynes (see picture). This reaction opens up a new catalytic route for the preparation of a class of potent organosilicon reagents for organic synthesis.

Multiple Pentafluorophenylation of 2,2,3,3,5,6,6-Heptafluoro-3,6-dihydro-2H-1,4-oxazine with an Organosilicon Reagent: NMR and DFT Structural Analysis of Oligo(perfluoroaryl) Compounds

The reagent Me3Si(C6F5) was used for the preparation of a series of perfluorinated, pentafluorophenyl-substituted 3,6-dihydro-2H-1,4-oxazines (2–8), which, otherwise, would be very difficult to synthesize. Multiple pentafluorophenylation occurred not only on the heterocyclic ring of the starting compound 1 (Scheme), but also in para position of the introduced C6F5 substituent(s) leading to compounds with one to three nonafluorobiphenyl (C12F9) substituents. While the tris(pentafluorophenyl)-substituted compound 3 could be isolated as the sole product by stoichiometric control of the reagent, the higher-substituted compounds 5–8 could only be obtained as mixtures. The structures of the oligo(perfluoroaryl) compounds were confirmed by 19F- and 13C-NMR, MS, and/or X-ray crystallography. DFT simulations of the 19F- and 13C-NMR chemical shifts were performed at the B3LYP-GIAO/6-31++G(d,p) level for geometries optimized by the B3LYP/6-31G(d) level, a technique that proved to be very useful to accomplish full NMR assignment of these complex products.

Nucleophilic Substitution at the 4‘-Position of Nucleosides: New Access to a Promising Anti-HIV Agent 2‘,3‘-Didehydro-3‘-deoxy-4‘-ethynylthymidine

For the synthesis of 2',3'-didehydro-3'-deoxy-4'-ethynylthymidine (8: 4'-Ed4T), a recently reported promising anti-HIV agent, a new approach was developed. Since treatment of 1-(2,5-dideoxy-beta-L-glycero-pent-4-enofuranosyl)thymine with Pb(OBz)4 allowed the introduction of the 4'-benzoyloxy leaving group, nucleophilic substitution at the 4'-position became feasible for the first time. Thus, reaction between the 4'-benzoyloxy derivative (14) and Me3SiCCAl(Et)Cl as a nucleophile led to the isolation of the desired 4'-"down"-ethynyl derivative (18) stereoselectively in 62% yield. As an application of this approach, other 4'-substituted nucleosides, such as the 4'-allyl (24a) and 4'-cyano (26a) derivatives, were synthesized using organosilicon reagents. In these instances, pretreatment of 14 with MeAlCl2 was necessary.

Synthesis of 4′-substituted cordycepins

Synthesis of 4'-substituted analogues of cordycepin was carried out. Electrophilic addition of N-iodosuccinimide/benzoic acid to 2',5'-bis-O-(tert-butyldimethyl-silyl)-3',4'-unsaturated adenosine (5) enabled the introduction of a leaving group to the 4'-position. The resulting product 7 was converted to 4'-(benzoyloxy) cordycepin (8) upon removal of the 3'-iodine atom. Compound 8 serves as a common precursor for the synthesis of the title compounds through nucleophilic substitution using organoaluminum and organosilicon reagents.

A NEW APPROACH TO THE SYNTHESIS OF 4′-CARBON-SUBSTITUTED NUCLEOSIDES: DEVELOPMENT OF A HIGHLY ACTIVE ANTI-HIV AGENT 2′, 3′-DIDEHYDRO-3′-DEOXY-4′-ETHYNYLTHYMIDINE

Oxidation of 3′-O-TBDMS-4′,5′-unsaturated thymidine 3 with dimethyldioxirane (DMDO) allowed the isolation of the epoxide 4. Upon reacting with organosilicon reagents in the presence of SnCl4, 4 underwent stereoselective ring opening to give 4′-α-allyl (6), 4′-α-(2-bromoallyl) (7), 4′-α-(cyclopenten-3-yl) (8), and 4′-α-cyano (9) derivatives of thymidine. Reactions of the 3′-epimer 12 with organoaluminum reagents gave 4′-α-methyl (13), 4′-α-vinyl (14), and 4′-α-ethynyl (15) analogues. Compounds 13–15 were transformed into corresponding 2′,3′-didehydro-3′-deoxy derivatives. Evaluation of their ability to inhibit the replication of HIV in cell culture showed that 4′-ethynyl-d4T (19) is more potent and less toxic than the parent compound d4T.

Synthesis of Organosilicon Reagents of Vinylsilicon Functionality via Cross‐Metathesis and Silylation of Alkenes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Nickel‐Catalyzed Cross‐Coupling of Organosilicon Reagents with Unactivated Secondary Alkyl Bromides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Organosilicon Reagents: Synthesis and Application to Palladium‐Catalyzed Cross‐Coupling Reactions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Neutral Coordinate‐Organocatalysts in Organic Synthesis: Allylation of Acylhydrazones with Allyltrichlorosilanes

AbstractN,N‐Dimethylformamide (DMF), sulfoxides, and phosphine oxides, etc., which are neutral, uncharged molecules and coordinate to silicon atoms of organosilicon reagents to activate nucleophilic addition, are defined as neutral coordinate‐organocatalysts (NCOs). In the presence of NCOs, allylation reactions of acylhydrazones, useful imine surrogates, using allyltrichlorosilanes proceed smoothly to afford the synthetically useful racemic and non‐racemic homoallylic amine derivatives in high yields with high diastereoselectivities.

Nickel-catalyzed cross-couplings of organosilicon reagents with unactivated secondary alkyl bromides.

A metal-catalyzed cross-coupling of organosilicon compounds with alkyl halides has been developed. Noteworthy attributes of the method are its scope (secondary electrophiles), its high functional-group compatibility, and the air stability of the catalyst components.

Iridium‐Catalyzed Mizoroki—Heck‐Type Reaction of Organosilicon Reagents.

Iridium‐Catalyzed Mizoroki—Heck‐Type Reaction of Organosilicon Reagents.

Ring opening of 4',5'-epoxynucleosides: a novel stereoselective entry to 4'-C-branched nucleosides.

Stereoselective synthesis of 4'-alpha-carbon-substituted nucleosides has been accomplished through epoxidation of 4',5'-unsaturated nucleosides with dimethyldioxirane (DMDO) and successive SnCl(4)-promoted ring opening of the resulting 4',5'-epoxynucleosides with organosilicon reagents. [reaction: see text]

Intramolecular anti‐Hydrosilylation and Silicon‐Assisted Cross‐Coupling: Highly Regio‐ and Stereoselective Synthesis of Trisubstituted Homoallylic Alcohols.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Intramolecular syn and anti hydrosilylation and silicon-assisted cross-coupling: highly regio- and stereoselective synthesis of trisubstituted allylic alcohols.

[reaction: see text] The geometrical isomers of 6-ethylidenedioxadisilacyclohexane were prepared by intramolecular hydrosilylation of an unsymmetrical disiloxane by the use of Pt (syn) and Ru (anti) catalysts. This new class organosilicon reagents underwent cross-coupling reactions with a range of aryl iodides to afford (E)- and (Z)-trisubstituted allylic alcohols in a highly stereospecific fashion.